One of the UK's best known former Army commanders in Iraq says he is "filled with horror" at the prospect of John Prescott running Britain this summer.
John Prescott: Traditionally stands in for Tony Blair
Mr Prescott usually takes the reins when Tony Blair is abroad on holiday but Colonel Tim Collins says he should not be left in charge of two wars.
His comments came as the prime minister refused to say directly whether his embattled deputy would take charge.
He also would not answer claims that Mr Prescott broke ministers' rules.
Parliament's sleaze watchdog this week extended his inquiry to cover allegations that the deputy prime minister failed to declare gifts.
Mr Prescott was reportedly given a "cowboy outfit" during a visit to the ranch of Philip Anschutz, the US tycoon who wants Britain's first super casino to be at the Millennium Dome.
There have also been calls for the prime minister to order an investigation into claims Mr Prescott's talks with Mr Anschutz broke rules on conflicts of interest.
Mr Prescott is currently in Turkey for the opening of the Baku-Ceyhan oil pipeline.
He insists he has done nothing wrong and says he was not involved at all in talks over the casinos.
But press reports say Labour chairman Hazel Blears and Chief Whip Jacqui Smith are being lined up to help him when Mr Blair is away.
And plans to send Mr Prescott "on tour" during Parliament's summer break are reportedly also being discussed.
Who's in charge?
Colonel Collins, who hit the headlines for his eve-of-war speech to troops in Iraq, voiced his dismay at Mr Prescott taking charge.
"I have to say that the prospect of John Prescott running the country in the absence of Tony Blair is a prospect that fills me with horror," he told the BBC's Daily Politics.
"A man who on the BBC last week by his own admission struggles with a paper and pencil, and is incompetent by his own admission with technology, running two complex wars, and a number of other complex issues in the nation leaves me wondering who actually is running the nation.
"Is it the civil service behind the scenes? Or... this man who by his own admission is not up to the job?
"It does fill me with horror, and I feel sorry for the troops who are deployed at the risk of their lives on his behalf."
At prime minister's questions, Tory leader David Cameron said Mr Prescott was apparently "really looking forward to standing in".
"Please tell us that isn't going to happen," pleaded Mr Cameron.
Mr Blair said the arrangements for the summer break would be "exactly the same as in previous years".
Downing Street always says the prime minister can be contacted if needed even when he is out of the country.
Tory MP Andrew Robathan suggested Mr Prescott had broken the ministerial code by having an affair with his diary secretary and accepting Mr Anschutz's hospitality.
He urged Mr Blair to call in his adviser on conflicts of interest, Sir John Bourn, to investigate.
The prime minister said it was "entirely right" to support the regeneration projects around the Dome, which is owned by Mr Anschutz.